Gas turbine engines and other turbomachines have rows of blades which rotate within a generally cylindrical case. As the blades rotate, their tips move in close proximity to the case. One way to improve the efficiency of such machines is to minimize the leakage of the working fluid between the blade tips and the case. As has been known for some time, this leakage may be reduced by blade and seal systems, in which the blade tips rub against an abradable seal attached to the interior of the engine case.
Porous metal structures are particularly useful for abradable seals, since they wear at a favorable rate when contacted by rotating blades. One method for making porous seals is to plasma spray a mixture of metal and polymer powder particles, generally according to the teachings of Longo in U.S. Pat. No. 3,723,165. However, when spraying a mixture of two or more types of powders as in Longo, it may be difficult to maintain the particles in a homogeneous mixture if the density or size of the particles differs, as discussed in U.S. Pat. No. 3,912,235 to Janssen. One attempt to overcome this problem is described in U.S. Pat. No. 4,386,112 to Eaton et al, wherein metal and ceramic powder particles are injected separately into the plasma stream, but in such a manner that the particles mix with each other in the spray stream. U.S. Pat. Nos. 3,020,182 to Daniels, 4,299,865 to Clingman et al, and 4,336,276 to Bill et al are also representative of the state of the art.
Notwithstanding the advanced state of plasma spraying technology, control over the quality and reproduceability of abradable seals applied according to prior art techniques has been difficult. Accordingly, improved methods of seal fabrication are sought.